Photons In Heavy-ion Collisions Research Articles

Interplay of effects of neutron skins in coordinate space and proton skins in momentum space on emission of hard photons in heavy-ion collisions near the Fermi energy

Interplay of effects of neutron skins in coordinate space and proton skins in momentum space on emission of hard photons in heavy-ion collisions near the Fermi energy

Feasibility of Measurements of the Properties of Thermal Photons in Heavy-Ion Collisions at the NICA Accelerator

Feasibility of Measurements of the Properties of Thermal Photons in Heavy-Ion Collisions at the NICA Accelerator

$$\alpha$$-clustering effect on flows of direct photons in heavy-ion collisions

In this work, we reconstruct the $\gamma$-photon energy spectrum which is in good agreement with the experimental data of $^{86}$Kr + $^{12}$C at $E/A$ = 44 MeV in the framework of our modified EQMD model. The directed flow and elliptic flow of free protons and direct photons have been investigated by taking $\alpha$-clustering structure of $^{12}$C into account. Comparing with the free proton, the direct photon flows give a clearer information about early stage of nuclear reaction. Difference of collective flows between different configurations of $^{12}$C is observed in this work. This indicates that collective flows of direct photons are sensitive to the initial configuration, therefore the $\gamma$ bremsstrahlung process might be taken as an alternative probe to investigate $\alpha$-clustering structure in light nucleus from heavy ion collisions at Fermi-energy region.

Emission of High-Energy Protons and Photons in Heavy-Ion Collisions Treated on the Basis of a Hydrodynamic Approach with a Nonequilibrium Equation of State

A hydrodynamic approach involving a nonequilibrium equation of state is used in describing heavy-ion collisions at medium and intermediate energies. The double differential cross sections for proton and photon emission in collisions of various nuclei are calculated in developing this approach with allowance for nuclear-viscosity effects and amendments introduced by the microcanonical distribution. The results agree everywhere, including the cumulative region of the spectrum, with available experimental data on the emission of high-energy particles. The approach in question permits reproducing high-momentum distributions of protons from the reaction $${}^{12}\textrm{C}+\,^{9}\textrm{Be}\to p+X$$ at the $${}^{12}$$ C ion energies of 300, 600, 950, and 2000 MeV per nucleon and the energy spectra of hard photons from the reaction $${}^{12}\textrm{C}+\,^{9}\textrm{Be}\to\gamma+X$$ at the $${}^{12}$$ C ion energies of 2.0 and 3.2 GeV per nucleon.

Recent results from PHENIX at RHIC

The PHENIX experiment at the relativistic heavy ion collider (RHIC) finished data taking in 2016. However, large datasets collected in different collision systems (p+p, p+A and A+A) at different energies (√sNN = 19-500 GeV) during the last years of the detector operation are actively analysed by the collaboration and bring a wealth of new experimental results. This paper reviews the most recent PHENIX results on the light flavour hadron production, yields and angular correlations of the direct photons in heavy-ion collisions as well as on the search for the onset of collectivity in high multiplicity p+p and p+A collisions.

Photons from thermalizing matter in heavy ion collisions

We investigate the production of direct photons in heavy ion collisions within the modified bottom–up thermalization scenario, which we show to be related to the thermalizing Glasma framework. The dynamics of the parton system up to thermalization/equilibration can be described by two momentum scales, by means of which we express the photon invariant yield excluding the prompt pQCD contribution. We derive an analytic formula, which provides an estimate of photon production from thermalizing matter for a wide range of collision systems and energies. We compare the yield with that measured in the PHENIX Au+Au run04 and the combined run07+run10 data sets at sNN=200GeV. We also make theory-data comparisons for Pb+Pb at 2760 GeV and d+Au at 200 GeV collision energies. Finally, we make predictions for the direct photon invariant yield from collisions of U+U, Cu+Au, He3+Au at 200 GeV and Pb+Pb at 5020 GeV.

Anisotropy of Photon Production: Initial Eccentricity or Magnetic Field

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of Relativistic Heavy Ion Collider show that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this Letter, we propose a method to experimentally measure the degree to which a magnetic field in heavy-ion collisions is responsible for the observed anisotropy of photon production. The experimental test proposed in this Letter may potentially change our understanding of the nonequilibrium stage and possible thermalization in heavy-ion collisions.

Photon azimuthal anizotropy and magnetic field in heavy-ion collisions

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of RHIC showed that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this talk, we consider a novel photon production mechanism stemming from the conformal anomaly of QCDxQED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. The comparison of the results to the data from the PHENIX collaboration, show that this mechanism might be the most responsible for the observed azimuthal anisotropy of photons.

Conformal Anomaly as a Source of Soft Photons in Heavy Ion Collisions

We introduce a novel photon production mechanism stemming from the conformal anomaly of QCD×QED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. This mechanism also provides a significant positive contribution to the azimuthal anisotropy of photons, v(2), as well as to the radial "flow." We compare our results to the data from the PHENIX Collaboration.

Conformal anomaly as a source of soft photons in heavy ion collisions

Conformal anomaly as a source of soft photons in heavy ion collisions

Elliptic flow of thermal photons in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

We calculate the thermal photon transverse momentum spectra and elliptic flow in $\sqrt{s_{NN}} = 200$ GeV Au+Au collisions at RHIC and in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions at the LHC, using an ideal-hydrodynamical framework which is constrained by the measured hadron spectra at RHIC and LHC. The sensitivity of the results to the QCD-matter equation of state and to the photon emission rates is studied, and the photon $v_2$ is discussed in the light of the photonic $p_T$ spectrum measured by the PHENIX Collaboration. In particular, we make a prediction for the thermal photon $p_T$ spectra and elliptic flow for the current LHC Pb+Pb collisions.

Quenching of hadron and photon spectra in heavy-ion collisions from RHIC to LHC

The generic features of parton energy loss effects on the quenching of single hadron spectra in heavy-ion collisions are discussed, paying attention to the expected differences from RHIC to LHC. The need for precise baseline measurements in p–Pb collisions at the LHC is also emphasized. Finally, I briefly mention the production of prompt photons in heavy-ion collisions as well as some open questions.

Measurements of thermal photons in heavy ion collisions with PHENIX

Thermal photons are thought to be the ideal probe to measure the temperature of the quark–gluon plasma created in heavy ion collisions. PHENIX has measured direct photons with pT < 5 GeV/c via their internal conversions into e+e− pairs in Au + Au collisions at GeV and has now provided a baseline measurement from p + p data.

Photon polarization as a probe for quark–gluon plasma dynamics

Prospects of measuring polarized photons emitted from a quark–gluon plasma (QGP) are discussed. In particular, the detection of a possible quark spin polarization in a QGP using circularly polarized photons emitted from the plasma is studied. Photons leave the QGP without further interaction and thus provide a primary probe for quark polarization within the QGP. We find that photon polarization cannot solely arise due to a possible QGP momentum space anisotropy, but may be enhanced due to it. In particular, for oblate momentum distributions and high photon energies, quark polarization is efficiently transfered to photon polarization. The role of competing sources of polarized photons in heavy-ion collisions is discussed.

Hanbury Brown–Twiss interferometry of direct photons in heavy ion collisions

We explore the possibility of measurements of Hanbury Brown–Twiss correlations in gg channels in the high multiplicity environment of heavy ion collisions. We consider all possible sources of photon-photon correlations and show that one can separate correlations of photons produced directly in the hot zone and residual correlations of decay photons: the former contribute to the region of small invariant relative momentum qinv&50 MeV, while the latter contribute to larger relative momentum and, negligibly, to the small qinv region. We make predictions of the photon correlation functions at SPS and RHIC energies.

The role of nuclear incompressibility in the production of hard photons in heavy-ion collisions

We summarize the properties of hard photons emitted in heavy-ion collisions. Dynamical phase-space calculations are used to describe the initial phase of the collision. Guided by the predictions, the photon spectra and the two-photon interference pattern are interpreted as a result of the collision dynamics and the photon spectra are viewed as an image of the phase space occupancy of participating nucleons. The data demonstrate the existence of two spatially and temporally distinct photon sources. It is shown how their relative intensity provides us with a measure of the incompressibility modulus of infinite nuclear matter.

Search for direct photons in heavy-ion collisions

A search for direct photon production has been performed using p and 16O projectiles at 60 and 200 A-GeV interacting with a C and a Au target. Photon and π 0 spectra have been measured in the pseudorapidity range 1.5 ⩽ η ⩽ 2.1 for the transverse momentum region 0.4 GeV/c ⩽ PT ⩽ 3 GeV/c with the lead-glass spectrometer SAPHIR. An upper limit of 15 % at the 90 % CL for the direct photon signal relative to the neutral pion production is obtained from the measured photon spectra and the Monte Carlo simulations of the hadronic background based on reconstructed π 0 and η spectra. Consequences for a possible phase transition to a quark-gluon plasma (QGP) are discussed.

Hard-photon production in heavy-ion collisions and the nuclear structure function

The production of hard photons in heavy-ion collisions at intermediate energies is studied. The particles result from first-chance single proton-neutron collisions. The cross sections are expressed in terms of the product of the structure functions of the nuclei. At large momentum transfers the structure function is related to the momentum distribution of the nucléons. The parametrization obtained from high-energy proton scattering gives a good description of the photon production data. The mass dependence of the reaction phase space and its implications are examined. An energy conservation condition that describes correctly the mass of the system is used. The Pauli principle is discussed. The experimental data are in agreement with the assumption that photons are produced before the colliding nuclei overlap.

Hard-photon production within a self-consistent transport approach to heavy-ion collisions

The dynamical evolution of nucleus-nucleus collisions is described by a transport equation of the Vlasov-Uehling-Uhlenbeck type where the time-dependent mean field and the in-medium nucleon-nucleon cross section is based on the same G-matrix. For numerical applications to heavy-ion reactions we employ a simple density-dependent parametrization for G, appropriate for nuclear configurations not too far from the ground state. We evaluate the radiative corrections to this G-matrix and compare with experimental hard-photon yields from nucleus-nucleus collisions. The origin of hard photons in heavy-ion collisions below 20 MeV/u is analyzed with respect to their contribution from first-chance NN collisions and from statistical emission of the highly excited fragments. We find that for nearly symmetric systems at these low energies both sources contribute with roughly the same order of magnitude. We, furthermore, investigate different parametrizations of G as obtained from high-energy p-nucleus data and confront the results for photon production with nucleus-nucleus data.

Energetic particle emission from nuclear co-operation

Abstract We discuss specific reaction mechanisms for the production of energetic particles such as pions and photons in heavy-ion collisions. Arguments from the uncertainty relation favour an interpretation in terms of co-operative multi-nucleon processes. We therefore analyze the implication of the co-operative model in more detail. The model takes into account the co-operation of many nucleons during the pion production process, and the formation of composite fragments along with the produced probe. The model is analyzed in terms of moving source parameters. Calculations are performed with the model and compared with recently measured data on pion production at a beam energy of 25 MeV nucl .